Re: [PATCH v4 3/3] rebase: set REF_HEAD_DETACH in checkout_up_to_date()

[John Cai <johncai86@gmail.com>]

Hi Junio,

On 17 Mar 2022, at 17:36, Junio C Hamano wrote:

> "John Cai via GitGitGadget" <gitgitgadget@gmail.com> writes:
>
>> From: John Cai <johncai86@gmail.com>
>>
>> Fixes a bug whereby rebase updates the deferenced reference HEAD points
>> to instead of HEAD directly.
>
> Perhaps
>
>     "git rebase A B", where B is not a commit, should behave as if
>     the HEAD got detached at B and then the detached HEAD got
>     rebased on top of A.  A bug however overwrites the current
>     branch to point at B, when B is a descendant of A (i.e. the
>     rebase ends up being a fast-forward).
>
>> ... See [1] for
>> the original bug report.
>
> OK (URL is wrong; see below).
>
> The explanation of how the bug occurs (elided) in the patch looked
> all reasonable.  It read well.
>
>> ...
>> Also add a test to ensure the correct behavior.
>
> Yup.  _Add_ a test to ensure that.  Not replace a misleading test
> that expected to see a wrong behaviour.
>
>> 1. https://lore.kernel.org/git/xmqqsfrpbepd.fsf@gitster.g/
>
> This is not the original bug report.  It was an early hint for
> diagnosis.
>
> [1] https://lore.kernel.org/git/YiokTm3GxIZQQUow@newk/
>
> would be a more appropriate pointer.
>
>>  	ropts.oid = &options->orig_head;
>>  	ropts.branch = options->head_name;
>>  	ropts.flags = RESET_HEAD_RUN_POST_CHECKOUT_HOOK;
>> +	if (!ropts.branch)
>> +		ropts.flags |=  RESET_HEAD_DETACH;
>>  	ropts.head_msg = buf.buf;
>
> OK.  If head_name is not set, we do not want to touch the branch
> the HEAD happens to be pointing at, so we want to detach.
>
>> +test_expect_success 'switch to non-branch detaches HEAD' '
>>  	git checkout main &&
>>  	old_main=$(git rev-parse HEAD) &&
>>  	git rebase First Second^0 &&
>> -	test_cmp_rev HEAD main &&
>> -	test_cmp_rev main $(git rev-parse Second) &&
>> -	git symbolic-ref HEAD
>> +	test_cmp_rev HEAD Second &&
>> +	test_cmp_rev main $old_main &&
>> +	test_must_fail git symbolic-ref HEAD
>
> As we want (1) HEAD (detached) is pointing at Second, (2) 'main'
> stayed at $old_main, and (3) HEAD is detched, these three conditions
> look sane.
>
> Thanks.
>
> For reference, I discarded [1/3], queued [2/3] and replaced this
> [3/3] with the following for now.

Sounds good--this is what I was about to do anyways!

>
> ---- >8 ---- ---- >8 ---- ---- >8 ---- ---- >8 ---- ---- >8 ----
> From: John Cai <johncai86@gmail.com>
> Subject: [PATCH] rebase: set REF_HEAD_DETACH in checkout_up_to_date()
>
> "git rebase A B" where B is not a commit should behave as if the
> HEAD got detached at B and then the detached HEAD got rebased on top
> of A.  A bug however overwrites the current branch to point at B,
> when B is a descendant of A (i.e. the rebase ends up being a
> fast-forward).  See [1] for the original bug report.
>
> The callstack from checkout_up_to_date() is the following:
>
> cmd_rebase()
> -> checkout_up_to_date()
>    -> reset_head()
>       -> update_refs()
>          -> update_ref()
>
> When B is not a valid branch but an oid, rebase sets the head_name
> of rebase_options to NULL. This value gets passed down this call
> chain through the branch member of reset_head_opts also getting set
> to NULL all the way to update_refs().
>
> Then update_refs() checks ropts.branch to decide whether or not to switch
> branches. If ropts.branch is NULL, it calls update_ref() to update HEAD.
> At this point however, from rebase's point of view, we want a detached
> HEAD. But, since checkout_up_to_date() does not set the RESET_HEAD_DETACH
> flag, the update_ref() call will deference HEAD and update the branch its
> pointing to. We want the HEAD detached at B instead.
>
> Fix this bug by adding the RESET_HEAD_DETACH flag in
> checkout_up_to_date if B is not a valid branch, so that once
> reset_head() calls update_refs(), it calls update_ref() with
> REF_NO_DEREF which updates HEAD directly intead of deferencing it
> and updating the branch that HEAD points to.
>
> Also add a test to ensure the correct behavior.
>
> [1] https://lore.kernel.org/git/YiokTm3GxIZQQUow@newk/
>
> Reported-by: Michael McClimon <michael@mcclimon.org>
> Signed-off-by: John Cai <johncai86@gmail.com>
> Signed-off-by: Junio C Hamano <gitster@pobox.com>
> ---
>  builtin/rebase.c  | 2 ++
>  t/t3400-rebase.sh | 9 +++++++++
>  2 files changed, 11 insertions(+)
>
> diff --git a/builtin/rebase.c b/builtin/rebase.c
> index b29ad2b65e..27fde7bf28 100644
> --- a/builtin/rebase.c
> +++ b/builtin/rebase.c
> @@ -829,6 +829,8 @@ static int checkout_up_to_date(struct rebase_options *options)
>  	ropts.oid = &options->orig_head;
>  	ropts.branch = options->head_name;
>  	ropts.flags = RESET_HEAD_RUN_POST_CHECKOUT_HOOK;
> +	if (!ropts.branch)
> +		ropts.flags |=  RESET_HEAD_DETACH;
>  	ropts.head_msg = buf.buf;
>  	if (reset_head(the_repository, &ropts) < 0)
>  		ret = error(_("could not switch to %s"), options->switch_to);
> diff --git a/t/t3400-rebase.sh b/t/t3400-rebase.sh
> index 6dc8df8be7..cf55b017ff 100755
> --- a/t/t3400-rebase.sh
> +++ b/t/t3400-rebase.sh
> @@ -389,6 +389,15 @@ test_expect_success 'switch to branch not checked out' '
>  	git rebase main other
>  '
>
> +test_expect_success 'switch to non-branch detaches HEAD' '
> +	git checkout main &&
> +	old_main=$(git rev-parse HEAD) &&
> +	git rebase First Second^0 &&
> +	test_cmp_rev HEAD Second &&
> +	test_cmp_rev main $old_main &&
> +	test_must_fail git symbolic-ref HEAD
> +'
> +
>  test_expect_success 'refuse to switch to branch checked out elsewhere' '
>  	git checkout main &&
>  	git worktree add wt &&
> --
> 2.35.1-757-g4266a5c05c

Thanks
John